At least i know now, why the new portal index page wasn't working for some users...
If simple portal and route changer are installed, the homepage becomes the default forum page and portal becomes forumurl/landingpage :/

Still not sure WHY they're conflicting, but they are definitely conflicting.
If i change the execution order from simple portal to be higher then route changer, it's working, but route changer isn't working anymore

it's posted in the route changer addon already
Let me see what this exactly mean and how we can fix this

That will make it so your portal page is at the index, and the old index page is at "forum".

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Note that this addon has a conflict with XenPorta. XenPorta has an option to swap out the index:

But the Route Changer addon overrides this option in XenPorta. That is why I added #2 feature above... to allow you to apply the same change in this addon. So if you use XenPorta and want to keep your portal at the index then you need to disable the above checkbox in XenPorta and then apply the route changes in the above example.